1. Field of the Invention
The present invention relates to antenna tracking systems for satellites and more particularly for a method and apparatus for tracking of multiple satellites using a single antenna.
2. Description of the Related Art
Satellite orbit estimation generally uses measurements made from one or more ground stations to determine the satellite orbit. These measurements include range from the ground station to the satellite, and the line-of-sight angles (typically Azimuth and Elevation) from the ground station antenna to the satellite. The preferred method for accurate angle measurement uses a monopulse or pseudo-monopulse tracking system that keeps the ground antenna pointed at a specific satellite using a signal transmitted by the satellite being tracked.
In the case of two or more collocated satellites, satellites located in close proximity defined by approximately ±0.05° of longitude, it may be possible to measure the range to each of the satellites but the ground antenna and its tracking system can be configured to provide Azimuth/Elevation angles for only one of the satellites at a time. The other satellites are generally within the ground antenna beam pattern and a single ground antenna can be used to transmit and receive signals to/from these other satellites but the line-of-sight angles to the other satellites cannot be measured from the single ground antenna.
Currently, the primary methods to address this problem include using multiple ground antennas (one per collocated satellite) to provide the angle data or time sharing a single antenna by tracking each of the collocated satellites in sequence.
These current methods are not advantageous for several reasons. Ground antennas with monopulse tracking are expensive to purchase and to maintain. In some cases there is not enough room at the earth station site for multiple antennas. There is also a requirement for accurate calibration of the tracking systems of the different antennas relative to one another—relative errors in the angle measurements for the different satellites can have a negative effect on the resulting orbit estimation accuracy. Similarly, time sharing a single antenna adds operational complexity and requires the tracking receiver to be retuned to a unique frequency for each collocated satellite. There is a possibility of error in identification of the resulting measurements and the possibility of pointing the ground antenna away from any of the satellites.
It is therefore desirable to provide a method and apparatus for using a single antenna for tracking of multiple substantially collocated satellites without requiring the complexities of time sharing.
It is also desirable that the method and apparatus provided be capable of retrofit to existing antenna stations.